Many thanks!
Upgrade is successful, and my PC detects Nano as removable disk. 
What have I done (this may help the others):
- start DSO with DOWN depressed
- strated DfuSe demo
… everything as before : PC can’t recognize DSO at USB, asks for some driver, DfuSe doesn’t detect DSO, etc …
(an old dancing with a tambourine) - I started C:\Program Files\STMicroelectronics\Software\DfuSe\Driver\dpinst_x86.exe
PC beeps shortly and miracle happens!